1. Field of the Invention
The present invention relates to a constant-current regulated power circuit for use, for example, in an ECL (emitter coupled logic), or the like.
2. Prior Art
FIG. 1 shows an ECL circuit using a conventional constant-current regulated power circuit. The circuit is a differential amplifier circuit in which respective emitters of a pair of bipolar transistors Q.sub.6 and Q.sub.7 are connected to each other so that currents flowing in the transistors Q.sub.6 and Q.sub.7 are made to flow into a constant-current regulated power circuit 30. In this circuit, a bipolar transistor Q.sub.8 is an emitter follower constituting an output stage. On the other hand, the constant-current circuit 30 includes a bias circuit 31 which is connected between the earth and a voltage source for supplying a voltage V.sub.cc so as to generate a predetermined voltage V.sub.o from the voltage supplied from the voltage source and the earth. The voltage V.sub.o is supplied to the base of a bipolar transistor Q.sub.1. The collector of the transistor Q.sub.1 is connected to the emitters of the transistors Q.sub.6 and Q.sub.7, and the emitter of the transistor Q.sub.1 is connected to the earth through a resistor R.
In this circuit, a current I expressed in the following equation flows into the collector of the transistor Q.sub.1 to which the voltage V.sub.o produced by the bias circuit 30 is supplied. EQU ={h.sub.fe /(1+h.sub.fe)}.times.{(V.sub.o -V.sub.BE)/R} (1)
As a result, a voltage V.sub.1 produced from the emitter of the transistor Q.sub.8 becomes as follows. EQU V.sub.1 =V.sub.cc -RI-V.sub.BE ( 2)
Here, when a temperature characteristic is necessary to the voltage V.sub.1 depending on an external specification, necessary constants including h.sub.fe, V.sub.BE, and R are selected, and the bias circuit 31 for producing the suitable voltage V.sub.o is designed. That is, to what value (central value) of the output voltage V.sub.o of the bias circuit 31 be set on the assumption that the V.sub.cc does not fluctuate, to what degree a temperature factor be set, and to what degree the dependency on the V.sub.cc fluctuation be suppressed, become important problems.
In designing the conventional bias circuit taking the above problems into consideration, however, there is a problem that even if a bias circuit stable against changes in temperature can be provided, the bias circuit is extremely unstable against changes in the supply voltage V.sub.cc.
Here, examples of the conventional bias circuit are shown in FIGS. 2(a) and 2(b) and FIG. 3. In the circuit shown in FIG. 2(a), a supply voltage V.sub.cc is lowered through a resistor R.sub.41, the lowered voltage V.sub.o is supplied to a collector of a bipolar transistor Q.sub.41 and is simultaneously divided by a resistor R.sub.42 between the collector and base of the transistor Q.sub.41 and a resistor R.sub.43 between the base and emitter of the transistor Q.sub.41 so as to cause a suitable current to flow into the transistor Q.sub.41. Accordingly, the output voltage V.sub.o is made stable.
FIG. 2(b) shows a circuit in which a supply voltage V.sub.cc is supplied to the collector and base of a bipolar transistor Q.sub.42 directly and through a resistor R.sub.44 respectively so that an output voltage V.sub.o is taken out from the emitter of the bipolar transistor Q.sub.42. The output voltage V.sub.o is supplied to the anode of a diode D through a resistor R.sub.45, the anode of the diode D being connected to the respective bases of bipolar transistors Q.sub.43 and Q.sub.44. The output voltage V.sub.o is supplied to both the respective collectors of the transistors Q.sub.43 and Q.sub.44 through a common resistor R.sub.46, and an earth level is supplied to both the respective emitters of the transistors Q.sub.43 and Q.sub.44 through a common resistor 47. Further, the collector of a transistor Q.sub.45 is connected to the base of the transistor Q.sub.42, the emitter of the transistor Q.sub.45 is connected to the earth, and the voltage V.sub.o is supplied to the base of the transistor Q.sub.45 through the resistor R.sub.46. In this bias circuit, the anode potential of the diode D changes in accordance with the fluctuation of the V.sub.o so that the currents flowing in the respective transistors Q.sub.43 and Q.sub.44 change, and hence the current flowing in the transistor Q.sub.45 changes as the current flowing in the transistors Q.sub.43 and Q.sub.44 changes, whereby the voltage V.sub.o is made stable.
Further, FIG. 3 shows a circuit in which a supply voltage V.sub.cc is lowered by a resistor R.sub.48 so as to obtain an output voltage V.sub.o. That is, the voltage V.sub.o is detected by a sense amplifier 41, and an output V.sub.2 in accordance with the fluctuation of the detected voltage is supplied to a current source 42 connected between a resistor R and the earth to thereby change a current value. Specifically, the circuit is of a negative feedback type in which when the voltage V.sub.o becomes high, the output V.sub.2 becomes high, so that an I.sub.o becomes high to thereby lower the V.sub.o to the original value. Accordingly, although the negative feedback operation is generated also against the fluctuation of the V.sub.cc so as to make it possible reduce the fluctuation of the output voltage V.sub.o, the fluctuation can not be suppressed to zero. Because the ratio of the fluctuation of the V.sub.cc to the fluctuation of the V.sub.o is determined based on the gain of the sense amplifier 41, the ratio of fluctuation can be made small by making the gain large. If the gain is made high, however, there is a risk of occurrence of oscillation. Then, although it is considered to provide a capacitor in the circuit to suppress the oscillation, not only the provision of such a capacitor brings an increase of the area an IC chip but there is a case where such a capacitance can not be formed depending on the size thereof.
Thus, the conventional constant-current regulated power circuits have a problem in that the output voltage V.sub.o of the bias circuit fluctuates in accordance with the supply voltage V.sub.cc, for example, practically, when the V.sub.cc fluctuates by 1 V, the bias voltage V.sub.o fluctuates by about 50 mV. Further, if it is desired that the circuit for suppressing the fluctuation is realized, there has been a problem in that the scale of the circuit becomes large-scaled.